The vertebral column, also called the backbone, is made up of 33 vertebrae that are separated by spongy discs and classified into five areas: (1) the cervical vertebrae which consists of seven bony parts in the neck; (2) the thoracic vertebrae which consists of 12 bony parts in the back area; (3) the lumbar vertebrae which consists of five bony segments in the lower back area; (4) the sacrum which consists of five sacral bones fused into one; and (5) the coccyx which consists of four coccygeal bones fused together into one. The five areas of the spine are shown in FIG. 1.
Lumbar disc disease occurs in the lumbar area of the spine. The lumbar area of the spine (and other areas of the spine) is made up of two parts:                vertebral bodies—the parts that are made of bone.        intervertebral discs—also known as simply as discs. The discs are located between the bony parts of the spine and act as “shock absorbers” for the spine.FIG. 2 shows an example of an intervertebral disc located between two vertebral bodies. The vertebral bodies are numbered from 1 to 5 in the lumbar area and the discs that are located between two of the vertebral bodies are numbered accordingly (e.g., L2-3, (or the disc located between vertebral bodies 2 and 3)).        
Each intervertebral disc is composed of two parts: (1) the annulus fibrosis—a tough outer ring of fibrous tissue, and (2) nucleus pulposus—located inside the annulus fibrosis. The nucleus is composed of a more gelatinous or soft material.
As humans age, the intervertebral disc may become dehydrated and compressed. This condition leads to the deterioration of the tough outer ring. FIG. 3 shows a healthy disc and a degenerated disc. As the tough outer ring degenerates, in some cases, the nucleus is pushed against the ring and eventually bulges out of the ring. This is considered a “bulging disc”. As the disc continues to degenerate, and with continued stress on the spine, the inner nucleus pulposus may actually rupture out from the annulus. This is considered a ruptured, or herniated, disc. FIG. 4a shows an example of healthy disc, and FIG. 4b shows an example of a herniated disc. The fragments of disc material can then press on the nerve roots that are located just behind the disc space. This can cause pain, weakness, numbness, or changes in sensation. Most disc herniations happen in the lower lumbar area, particularly in the L4-5 and L5-S1 areas. FIG. 5 shows a herniated disc, while FIG. 6 shows an example of a healthy disc. Note the uneven distribution of forces on the herniated disc.
Lumbar disc disease is due to a change in the structure of the normal disc. Most times, disc disease is the result of aging and the degeneration that occurs within the disc. Occasionally, severe trauma can cause a normal disc to herniate and trauma may cause an already herniated disc to worsen.
Spinal fusion is a surgery that fuses vertebrae together. Typically, two vertebrae are permanently coupled so that there is no longer any movement between them. In some cases, the surgeon will use a graft (such as bone) to hold (or fuse) the bones together permanently. There are several different ways of fusing vertebrae together. In one example, strips of bone graft material may be placed over the back part of the spine to fuse two vertebrae together. In another example, the bone graft material is placed between the vertebrae. In yet another example, a special cage is placed between the vertebrae and the cage is filled with bone graft material. In further examples, the vertebrae are fused together using screws, plates, and/or cages.
There are many disadvantages associated with spinal fusion. Spinal fusion is designed to eliminate the normal motion of one or more lumbar segments in the spine. Accordingly, the spinal column above and below the fusion area is more likely to be stressed when the spine does move. Thus, persistent stress can cause future problems in un-fused areas of the spine.
Disc nucleus replacement is a procedure that replaces the soft jelly center of the natural disc (or a portion thereof) with a prosthetic disk nucleus (PDN) such as an artificial gel sac. The gel sac alleviates pain and further damage by acting as a shock absorber that prevents the spine from applying pressure to the nerves. Another potential benefit of the gel space is that it allows more movement of the spine, and therefore prevents disk degeneration below and above the site of surgery. As a result, the gel sac allows the cartilage surrounding the nucleus to heal and the patient can resume normal activity. Disc nucleus replacement surgery can be performed using a minimally invasive laparoscopic procedure, which is performed through tiny cuts using miniature tools and viewing devices.
One example of an in-situ curable polyurethane nucleus replacement device is the DASCOR™ Disc Arthroplasty device. The DASCOR™ device is made by mixing two-parts of liquid polymer while delivering it through a catheter to an expandable polyurethane balloon that is placed in the disc space. The polymer cures in a matter of minutes, changing state from a liquid to a firm, but pliable solid device. After 15 minutes, the delivery catheter is removed, leaving the final implant device. The balloon catheter has a low profile and can be inserted into the disc space through a small annulotomy (e.g., 5.5 mm). The mixed liquid polymer is delivered to the balloon under controlled pressure, causing the balloon to expand to contour and fill the entire disc space left by the nucleotomy procedure.
The use of the in-situ curable device provides for implantation of a large volume device through a small annulotomy, thus making migration of the solidified device unlikely. Additionally, the system has the versatility of creating an implant of whatever size that is created by the nucleotomy.
Also, the device can be used in combination with other components such as endplates that are affixed to the vertebrae. In particular, the deployment of a large and pliable device located between the endplates and that contours to the endplates can help balance associated load transfer between the annulus and the artificial nucleus while minimizing endplate disruption.
Another related advantage of the in-situ curable device is its ability to generate distraction forces inside the nucleotomy space. Therefore, the implantation of the device not only offers the ability to fill any given space left by nucleotomy, but also the potential to distract and restore a collapsed intervertebral disc.
A disadvantage associated with the in-situ curable device is that the polymer might not be robust enough over time to support the compressive loads of the spine.
Total disc replacement is another example of a spinal surgery. In some cases, the entire disc is beyond repair and a complete disc replacement is necessary. In such an instance, total disc replacement can be performed instead of spinal fusion surgery. Nonetheless, total disc replacement has not yet been shown to be superior to spinal fusion. Total disc replacement involves replacing the disc with an artificial disc. Some artificial discs (such as ProDisc, Link, SB Charite) consist of two metal plates and a soft core.
The SBCharité III is an example of an artificial disc used to replace an entire disc. The SBCharité III is composed of two endplates of high quality cobalt chromium alloy. The endplates are coated with titanium and a hydroxyapatite porous coating to enhance bone fixation (osteointegration). The endplates are fixed to each vertebrae using anchoring teeth along the edges of the plates. The natural movement of the disc is made possible with a dense polyethylene sliding core that is placed between the endplates. In this manner, the core acts as a spacer to maintain a natural distance between the two vertebrae and also more naturally supports the spinal column.
Unlike spinal fusion, disc replacement technologies (such as the DASCOR™ and the SBCharité III) do not require grafts and provide for a more natural movement of the spine so that further injuries to the spine are diminished. To this end, disc replacement technologies attempt to restore and maintain normal physiological motion. This is accomplished by (1) restoring and maintaining a natural intervertebral separation height, (2) restoring and maintaining a natural lordosis, (3) restoring and maintaining a natural instantaneous axis of rotation; (4) correcting abnormal motion; (5) reducing or eliminating pain in the spine, and (6) improving functional ability of the patient. If these goals are achieved, the segments of the spine adjacent to the artificial disc will be free of abnormal loads and motions. Accordingly, there would be a deceleration or elimination of stress applied to spine segments adjacent the artificial disc.
There are some disadvantageous associated with current disc replacement technologies. Artificial discs that use polymer materials tend to degenerate because polymer strength diminishes over time, especially under loads, a phenomenon known as creep. As the polymer materials degenerate, the core between the endplates of the artificial disc will wear thin, changing the intervertebral distance and causing wear debris to undesirably migrate into the spinal area. The patient may react to this debris with an inflammatory response that can cause pain, implant loosening, and further implant failure.
The artificial disc device itself may also be a source of complications. The device can shift out of its normal position and even dislocate. If the device migrates out of position, it can cause injury to the nearby tissues. A second surgery may be needed to align or replace the device. Similar to other types of joint replacements, the artificial disc device may fail over time as its components degenerate. An artificial disc device is estimated to last 15 to 20 years. Once the device fails, it is removed and typically replaced with spinal fusion surgery.
Subsidence is another possible problem of artificial disc devices. Subsidence happens when the disc device sinks down into the vertebral body or is pushed up into the vertebral body. Subsidence can result in a loss of the normal disc height, which, in turn, could result in the compression of nerves and adverse neurological symptoms.